1. Field of the Invention
The present invention relates to a voice band expander and expansion method and a voice communication apparatus that enhance a band-limited voice signal by adding high frequency components not present in the band-limited voice signal.
2. Description of the Related Art
Telephone transmission has traditionally been limited to the frequency band from 300 Hz to 3,400 Hz. Although this limited frequency band permits intelligible voice communication, the quality of the reproduced voice signal is unsatisfactory, and sometimes the voice signal is not reproduced clearly enough to be easily comprehended.
Various attempts have been made to solve this problem by band expansion, that is, by adding frequencies above 3,400 Hz or below 300 Hz to the reproduced signal. In Japanese Patent Application Publication No. 2002-82685, for example, Tokuda describes a band expansion method in which a band-limited voice signal is folded over to generate high frequency components that are added to the band-limited voice signal as shown in FIGS. 1A and 1B. In these drawings Fs represents the sampling frequency of the telephone equipment. Fs/2 is the upper limit of the band-limited signal and the center of symmetry of the foldover process.
There are, however, two problems with this foldover method.
One problem is related to the resonant frequency components of a voice signal referred to as formants. In general, formants produce a spectral envelope with pronounced peaks and troughs, as exemplified by the dotted line in FIG. 1A. If this spectral shape is directly folded over into the higher frequency band above the limited voice band), it produces peaks that were not present in the high-frequency spectrum of the original voice signal, resulting in a reproduced voice signal distorted by extraneous resonances.
The other is a problem of harmonic frequency structure. The harmonic frequency structure of a voice signal, indicated schematically by the solid lines in FIG. 1A, reflects the pitch of the speaker's voice. This harmonic structure is also present in the high frequencies excluded from the limited voice band, but at a lower intensity. The harmonic structure of the foldover components generated in the higher frequency band by the technique disclosed by Tokuda has too high an intensity: the higher harmonics fail to decay properly, resulting in an unnaturally shrill reproduced voice signal.
An alternative to the foldover method is frequency shifting, in which the band-limited frequency spectrum is shifted or copied directly into the higher frequency band above the limit frequency, but this method fails to solve the above two voice quality problems.
The invention also provides a voice band expander using the invented method, and a communication apparatus using the voice band expander.